1. Field of the Invention
The present invention relates to RFI (Radio Frequency Interference) cancellers for use in subscriber line transmission systems, subscriber line transmission systems incorporating RFI cancellers, receivers, incorporating RFI cancellers, for use with subscriber line transmission systems, and methods of suppressing RFI in telecommunications wire transmission systems employing multi-carrier modulation. The invention has particular application to VDSL systems employing DMT.
2. Discussion of the Background
The frequency range occupied by VDSL contains several frequency bands, the HAM bands, which are reserved for amateur radio users. VDSL signals on an unshielded twisted wire pair will be disturbed by these radio frequency signals, through common mode (CM) to differential mode (DM) conversion. The commonest example of an RFI signal is the disturbance that VDSL systems receive from a nearby HAM radio transmitter. An analogue RFI canceller, that uses the coupling between CM and DM, can be used to bring the RFI down to a level where it does not saturate the A/D converter in the VDSL receiver. But even if an analog RFI canceller is used, the level of disturbance left can still severely damage the performance of a VDSL system.
The present invention seeks to reduce the RFI in the frequency domain of a DMT based VDSL system after A/D conversion.
Frequency domain Radio Frequency Interference (RFI)-cancellation in the present invention uses a series expansion of the transfer function from the RFI disturbance to the DMT carriers.
Although the present invention is described in relation to VDSL systems employing DMT, it will be apparent to those skilled in the art that the present invention can also be applied to ADSL and related systems, using any multi-carrier modulation method. The necessary modifications of the present invention needed to apply it to alternative subscriber line transmission systems using multi-carrier modulation, will be immediately apparent to those skilled in the art.
According to a first aspect of the present invention, there is provided a RFI canceller, for use in a subscriber line system using multi-carrier modulation, characterised in that measurement means are provided for measuring a RFI disturbance in carriers falling within a band of frequencies causing said RFI, in that estimation means are provided for estimating RFI ingress into carriers outside said band of frequencies, and in that adder means are provided for subtracting an error correcting signal derived from said estimation means from a received signal.
Preferably, said subscriber line system is a VDSL system and said multi-carrier modulation is DMT.
Said RFI canceller includes:
a demodulator means for demodulating an incoming data stream to provide a first parallel data stream;
a parallel to serial convertor for converting said first parallel data stream to a first serial data stream;
a digital to analogue convertor for converting said first serial data stream to a first analogue signal;
an analogue RFI canceller circuit for combining said analogue signal with an analogue error correcting signal, to produce a second analogue signal;
an analogue to digital convertor to convert said second analogue signal to a second serial data signal;
a serial to parallel convertor for converting said second serial data stream to a second parallel data stream;
a modulator means for modulating said second parallel data stream onto a multiplicity of carriers; and
a digital RFI canceller means including said measurement means, said estimation means and said adder means.
Equaliser means may be connected to an output of said digital RFI canceller means.
Said demodulator means may be adapted to perform an inverse discrete Fourier transformation on an incoming digital signal, and said modulator means may be adapted to perform a discrete Fourier transformation on an outgoing digital signal.
Said band of frequencies may be narrow compared to a band of frequencies occupies by said multi-carriers.
Said band of frequencies may correspond to the HAM band.
Said measurement means may operate on at least one carrier within said band of frequencies.
Said estimation means may approximate a transfer function:             G      k        ⁢          xe2x80x83        ⁢          (      f      )        =            1      -              ⅇ                  j          ⁢                      xe2x80x83                    ⁢                                    4              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              fN                                      f              s                                                  1      -              ⅇ                  j          ⁢                      xe2x80x83                    ⁢                      (                                          2                ⁢                                  xe2x80x83                                ⁢                π                ⁢                                  xe2x80x83                                ⁢                                  f                                      f                    s                                                              -                                                π                  N                                ⁢                                  xe2x80x83                                ⁢                k                                      )                              
from the RFI disturbance to the DMT carriers, by a linear combination of a predefined set of basis functions.
Said estimation means may approximate only the denominator of Gk(f), namely:   1      1    -          ⅇ              j        ⁢                  xe2x80x83                ⁢                  (                                                    2                ⁢                                  xe2x80x83                                ⁢                π                ⁢                                  xe2x80x83                                ⁢                f                                            f                s                                      -                                          π                N                            ⁢                              xe2x80x83                            ⁢              k                                )                    
Said measuring means may only perform measurements on carriers having frequencies that are not close to a centre frequency of the RFI disturbance.
Said basis functions may be polynomials.
Said basis functions
xcfx86k,n:kxcex5M∪U
may bee defined by a linearized model of said RFI disturbance             S      k        ≈                  S        ^            k        =            ∑              n        =        1                    L        +        M              ⁢          xe2x80x83        ⁢                  α        n            ⁢              xe2x80x83            ⁢              ϕ                  k          ,          n                    
where xcex1n are unknown coefficients calculated by using a least-squares fit technique.
Said estimation means may derive said basis functions by means of a Taylor expansion from a linearized model of said RFI disturbance, namely:             S      k        ≈                  S        ^            k        =                    ∑                  l          =          0                          L          -          1                    ⁢              xe2x80x83            ⁢                        A          l                ⁢                  xe2x80x83                ⁢                  D          k                      (            l            )                          ⁢                  xe2x80x83                ⁢                  (                      -                          f                              c                _                                              )                      +                  ∑                  m          =          0                          M          -          1                    ⁢              xe2x80x83            ⁢                        B          m                ⁢                  xe2x80x83                ⁢                  D          k                      (            m            )                          ⁢                  xe2x80x83                ⁢                  (                      -                          f                              c                _                                              )                    
where {Al}∪{Bl} is a set of L+M unknown parameter coefficients calculated by said estimation means.
No more than one thousand basis functions may be employed.
No more than ten basis functions may be employed.
According to a second aspect of the present invention, there is provided a subscriber line system using multi-carrier modulation, characterised in that said subscriber line system includes at least one RFI canceller as setforth in any preceding paragraph.
Said subscriber line system may be a VDSL system.
Said subscriber line system may be an ADSL system.
Said multi-carrier modulation may be DMT.
According to a third aspect of the present invention, there is provided a receiver for use with a subscriber line system, as set forth in any preceding paragraph, characterised in that said receiver includes a RFI canceller as set forth in any preceding paragraph.
According to a fourth aspect of the present invention, there is provided a method of reducing RFI in a telecommunications wire transmission system employing multi-carrier modulation characterised by:
measuring a disturbance signal induced by RFI in carriers falling within a band of frequencies causing said RFI;
estimating RFI ingress into carriers outside said band of frequencies; and
subtracting an error correcting signal, derived from an estimation of said RFI ingress, from a received signal.
Said telecommunications wire transmission system may be a VDSL system.
Said telecommunications wire transmission system may be an ADSL system.
Said multi-carrier modulation may be DMT.
Said method may include the steps of:
demodulating an incoming data stream;
converting demodulated data to a first serial data stream;
converting said serial data stream to a first analogue signal;
combining said first analogue signal with an analogue error correcting signal, to produce a second analogue signal;
converting said second analogue signal to a second serial data stream;
converting said second serial data stream to a parallel data stream;
modulating said parallel data stream onto a multiplicity of carriers; and then
measuring a disturbance induced by RFI in carriers falling within a band of frequencies causing said RFI;
estimating RFI ingress into carriers outside said band of frequencies; and
subtracting an error correcting signal, derived from an estimation of said RFI ingress, from a received signal.
An outgoing data stream may be equalised.
An inverse discrete Fourier transformation may be performed on an incoming digital signal to demodulate said incoming digital signal, and a discrete Fourier transformation may be performed on an outgoing digital signal to modulate said outgoing signal.
Said band of frequencies may be narrow compared to a band of frequencies occupies by said multi-carriers.
Said narrow band of frequencies may correspond to the HAM band.
Said measurements may be performed on at least one carrier within said band of frequencies.
Said step of estimating may approximate a transfer function:             G      k        ⁢          xe2x80x83        ⁢          (      f      )        =            1      -              ⅇ                  j          ⁢                      xe2x80x83                    ⁢                                    4              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              fN                                      f              s                                                  1      -              ⅇ                  j          ⁢                      xe2x80x83                    ⁢                      (                                          2                ⁢                                  xe2x80x83                                ⁢                π                ⁢                                  xe2x80x83                                ⁢                                  f                                      f                    s                                                              -                                                π                  N                                ⁢                                  xe2x80x83                                ⁢                k                                      )                              
from the RFI disturbance to the DMT carriers by a linear combination of a predefined a set of basis functions.
Only the denominator of Gk(f) may be approximated, namely:   1      1    -          ⅇ              j        ⁢                  xe2x80x83                ⁢                  (                                                    2                ⁢                                  xe2x80x83                                ⁢                π                ⁢                                  xe2x80x83                                ⁢                f                                            f                s                                      -                                          π                N                            ⁢                              xe2x80x83                            ⁢              k                                )                    
Measurements may be performed only carriers having frequencies that are not close to a centre frequency of said RFI disturbance signal.
Said basis functions may be polynomials.
Said basis functions
xcfx86k,n:kxcex5M∪U
may be defied by a linearized model of said RFI disturbance             S      k        ≈                  S        ^            k        =            ∑              n        =        1                    L        +        M              ⁢          xe2x80x83        ⁢                  α        n            ⁢              xe2x80x83            ⁢              ϕ                  k          ,          n                    
where xcex1n are unknown coefficients calculated by using a least-squares fit technique.
Said basis functions may be derived from a Taylor expansion of a linearized model of said RFI disturbance, namely:             S      k        ≈                  S        ^            k        =                    ∑                  l          =          0                          L          -          1                    ⁢              xe2x80x83            ⁢                        A          l                ⁢                  xe2x80x83                ⁢                  D          k                      (            l            )                          ⁢                  xe2x80x83                ⁢                  (                      -                          f                              c                _                                              )                      +                  ∑                  m          =          0                          M          -          1                    ⁢              xe2x80x83            ⁢                        B          m                ⁢                  xe2x80x83                ⁢                  D          k                      (            m            )                          ⁢                  xe2x80x83                ⁢                  (                      -                          f                              c                _                                              )                    
where {Al}∪{Bl} is a set of L+M unknown parameter coefficients and said set of L+M parameters may be calculated.
No more than one thousand basis functions may be employed.
No more than ten basis functions may be employed.